1. Field of the Invention
The present invention relates to a developing apparatus used in an image forming apparatus such as an electrophotographic copying machine or a laser printer to develop an electrostatic latent image formed on the surface of a photosensitive drum serving as an image carrier by using a one-component developer.
2. Description of the Related Art
Electrophotography is widely used as a technique of printing images on plain paper in a copying machine or a laser printer. Development systems in electrophotography are roughly classified into liquid development and dry development. Dry development is generally employed. Dry development systems are also classified into a two-component developer system and a one-component developer system. Recently, the one-component developer system has become increasingly popular especially in compact printers and compact copying machines.
In the one-component developer system, a developer contains no carrier (e.g., an iron powder or a ferrite powder), and the structure of a developing apparatus is simpler than that in the two-component developer system. For this reason, the one-component developer system is more advantageous in reducing the size and cost of a developing apparatus.
As one of such one-component developer systems, an impression development system is known. This system is characterized in that a toner or a toner carrier is brought into contact with an electrostatic latent image on a photosensitive drum at a substantially zero relative peripheral speed (see U.S. Pat. No. 3,152,012 and Jpn. Pat. Appln. KOKAI Publication Nos. 47-13088, 47-13089 and 3-102372, and the like).
According to this system, since no magnetic material is used, a developing apparatus and an image forming apparatus can be simplified and reduced in size. In addition, a color toner can be easily used.
Although a developing apparatus of the one-component developer system is advantageous in terms of simplification and reduction in size of an image forming apparatus, the one-component developer system is inferior to the two-component developer system in terms of the charging characteristics (rising characteristics) and conveying characteristics (conveying amount) of a toner. This is because the toner in the one-component developer system has no carrier for charging the toner and conveying the toner onto a developing roller. Especially when the conveying amount of the toner is smaller than the amount of the toner consumed in development, a decrease in image density occurs on a printed image.
For example, in printing a graphic pattern, which has recently become popular, since there are many solid portions (image portions having relatively large areas to which toner particles are attached), stabilization of the toner conveying amount is indispensable. For this purpose, in some case, a former or a developer supply roller (simply called an intermediate roller in many instances) is arranged to be in contact with a developing roller or brought into contact therewith with a predetermined bite amount so as to stably supply a toner onto the developing roller, and a certain potential difference is set between the applied voltage (developing bias voltage) to the developing roller and the developer supply roller so as to electrically convey the toner to the developing roller.
This developer supply roller generally consists of a foamed elastomer (so-called sponge) because of the necessity to protect the surface of the developing roller and convey a large amount of a toner. With such a roller, a stable toner conveying operation can be performed for a while after the use of the developing apparatus is started. However, as the developing apparatus is used, toner particles enter the foamed elastomer (to be simply referred to as the sponge hereinafter) to increase the hardness, reduce the outer diameter, and form a skin-like film on the sponge surface. As a result, the toner conveying amount (toner supply amount) of the sponge decreases as compared with its initial state.